Looking For Ideas For An Excavation

guyute65045

I am getting ready to bury a 10,500 gal water tank for a fire suppression system. The tank is going in the ground 9' from an existing building, the top of the footing is 5'8" below grade(1 foot footing). The dimension of the tank is 13'2" top to bottom, with either 12" of stone under it or on undisturbed soil, there is a 4' round sump station located 5-10' away from it/building that is 4'8" deeper than the bottom of the tank.
My question is this: the tank was supposed to go in back in the early part of sept when the water table was much lower, but the engineers and precast co have been redesigning if for the last month and a half and the water table is up substantially, finish grade on top of the tank is 410' and the water table is at 394.
I need to make sure that the building does not shift but with the water up I am worried about the 45* slope leading up to the existing footing is going to undermine. Driving sheet piling will cost 22K, I was going to underpin the corner originally, but with the water up that doesn't seem like a good idea. I obviously will de water and drop the table some, but am nervous about the proximity of the building. It is up to me for BMP to design the support.
Any ideas?
to add insult to injury the engineers wont answer the questin how much ground cover over the tank. I guess it is not that important

Thanks in advance.
John

tgeb

Two options come to mind;

Move the excavation further away from the foundation.

Install 2 smaller tanks.

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cexcavation

How long is this thing? If it is not too long, you may be able to build some sort of shoring by driving a few pilings to prevent shifting. From there, anchor the pilings back towards the building. Erosion is mainly what you would be fighting if the soil is native compacted. If you can keep the bottom from blowing out, you should be fine. Set the tank, and then backfill a few feet at at time. Pull your shoring up in increments as you backfill. Once backfilled, pull the pilings out (could be 3" sch 80pipe with tapered ends). All I can say is I am glad it is you and not me. Precision work and high water tables should not be said in the same sentence.

Good Luck!!!

oldfrt

Probably know this already,but that tank should be filled first if backfilling with wet soil.
Or she'll pop up like a fart in a bathtub.

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Kgmz

How big is this tank?

Width and length, already know it is 13'2" tall. I am guessing 10' wide x 12' long x 12' high internally.

How much area is there to put in a tank?

Just wondering if there were space constraints, and that is why you went with a precast tank. If there is enough space I would have gone with Xerxes or Containment Solutions fiberglass tank. Then you wouldn't have to go quite as deep, and much easier to install.

Also is this a drive-on tank. Strong top able to take the weight of a vehicle driving over it, or are they going to do something so this could never happen.

What kind of soils?

This answer will help with your question, and I would use a slide rail shoring system instead of sheet piling. A lot less money to rent and you slide it down and add panels as you dig it out, and no need to slope the excavation.

PipeGuy

What is "it"? A 12' diameter precast manhole with an anti-flotation base? A vault? If I'm understanding right you need to dig a hole about 18' deep only 9' away from a 6' deep footing.

I just got a quote for a 30K gallon fiberglass cistern from here http://www.darcoinc.com/fire-cisterns.html . It seemed reasonably priced. A 10K model might save you several vertical feet of excavation and eliminate the need for a big crane. Maybe you could stick-sheet the excavation with timber walers and random width boards and leave it in-place upon backfill? In decent soil you oghta' be able to that pretty quick.

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rino1494

Drive in some 6x6 posts and stick a road plate in front of them or some 3/4" plywood.

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cexcavation

Hey no stealing ideas Never mind, I was way more high tech with the sch 80 pipe vs. the 6 by 6 posts. Either way, I think it would work if a guy didn't dink around and leave it open for days.

dayexco

have a well driller pop you a couple of wells in order to get rid of the water, get some 20' long sched 80 spreaders for your trench box, weld some plate on the open ends...start digging inside it, pushing it down as you go. most boxes are 8' tall, so you'd only have to concern yourself with the 45 degree taper from the top of the box up to the existing ground elevation. done it quite a few times.

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guyute65045

To answer a previous question the tank is 16' long 8' wide, 13' 2" high. the soils are cobbly gravel as it is uphill from a river. I dont think the ground water comes into play up there until the very bottom of the excavation for the tank. The sump chamber is another story as it is almost 5' lower, but it is detached and 5-10ft away from the far end of the tank.


existing -
- O ----------sump
----------- * Gah! this drawing worked before.
[ * * musta got reformatted
[ * *
[ * *
[ * *
[ * < ------Tank
[
[
--------------------------------------------
-
New addition -
-



that is the proposed layout, like I said the redesign is not even done yet.
I like the ideas so far, one thing I thought of asking was if I could replace any of the load bearing soil that gets undermined with flowable fill. It is really the 45* angle of load bearing soil that I am most worried about.

thanks for all of the input

Kgmz

At least you will have the narrow end of the tank by the building. But I would still use a slide rail system to minimize excavation, if you slope it that is a lot more dirt.

Found this video that shows a typical slide rail system. They also make a clear span sytem without the roller bar in the middle for long excavations that require a upright in the middle.

http://video.google.com/videoplay?do...3230781987364#

nedly05

If the cobbly river gravel that you have to dig in is anything like it is over here, you will end up with a hole at is much larger than you need. That type of ground just doesnt hold up well during excavation, the banks cave constantly.

Kgmz

I thought that same thing. Thats why I wouldn't do this without some type of very sturdy shoring all the way to the top, no sloping. Sometimes that stuff is like digging in a bowl of marbles. We have a lot of that around here.

Little

I personally love to use the slide rail boxes, they are great when doing these types of large tanks. The engineer isn't telling you any specifics of what they want for the existing buildings footing for support? Also, def recommend you establish a dewatering well of some sort with a large pump. Fortunately i have access to a 6" centrigual diesel pump that pumps ungodly amounts of water hehe.

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guyute65045

Just got the redesign bak and it looks like the new tank 11x15x9 will keep me up out of the water and away from the load bearing soil. I guess that they didnt like the number I gave to shore up the excavation.

cexcavation

It's funny how engineers often overlook the complexities of performing the work they design. It is also funny that it takes a lowly excavator to point it out in order to cause the design to actually become cost effective.

Why do engineers make the big bucks?????? I was always told it was because they save you money in the long run. I say it is the tradesmen who should make the big bucks, as we are typically the ones uncovering the pitfalls of designs with regards to the financial burden on the owner.

Way to go on the reality check Guyute!!!

Little

Good deal, thats why i was kinda wondering if the engineer was looking to add any input to the matter.

I find it to be incredible the amount of mistakes you will find on a set of big site plans made by the engineers. I mean the plans are even signed off by the chief engineer after they are drawn up by the drafter. The second thing that always bugs me is that these guys draw up these insane contour lines, we just look at them and laugh.... ok so you want the water to go that way, ok lets see, lets take this contour line with its 100 bends and jigs and jags and make one nice straight one. Aaaaaaah, nice.

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dayexco

well...i'm often very flattered because i have local seasoned engineers.."btw, 3 of my brothers are engineers"....call me, and say....hey, i'm going to email you plans for this project......this is what we want/need to do....this is what we'd like it to look like, this is what it needs to do....can you build it...what do we need to tweek to make it work for both of us? you need to work with these guys...get into a good working relationship with them. word gets around you're butt blasting their design....they'll work around you...trust me, i've seen it happen....they're not your enemy...meet them 40% of the way, they're climb mountains to get the other 60%

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cexcavation

Thanks for making that clear Gene. That is one thing that I am good at doing.......playing well with others!!! The point I was making was that if the engineer doesn't consult the excavator/builder/etc. prior to the sign off on the plans, things often end up costing more.

I had to replace an entire sewer lateral and pull up the outlet on an existing house, etc. due to an engineer not accounting for proper fall when designing the new main. Not a big deal, but I asked him later why he didn't check any of the as built elevations. His response was that he wasn't worried since you could always just pump it..........this was said right in front of the developer that hired him to save himself money.

Overall, if possible, work with the engineer prior to starting any project to go over any pitfalls and expensive change orders. I know the tendency is to bash on engineers, but they have their area of expertise and we have ours. Combine the two and everyone wins. Occasionally you have the engineer that can never be wrong, and that is where all you can do is just smile, nod, and document everything. My best friend is a Civil Engineer, so I like to run things by him on a regular basis.

PipeGuy

Paraphrasing something I read somewhere.......
Architects know a little about a lot. Engineers know a lot about a little. Contractors, by virtue of their working closely with architects and engineers, know less and less about more and more until they eventually know nothing about everything.

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